The invention relates to a printable, transparent, coextruded polyolefin multilayer film which is low-temperature sealing on both sides. The multilayer film is distinguished by low film turbidity, increased scratch resistance, high surface gloss, printability, low sealing initiation temperatures (low-temperature sealing) on both sides, good slip properties, and universal usability on high-speed packaging machines. The invention also relates to a process for the production of the film and its use.
DE-A-16 94 694 discloses laminated materials which are sealable at high temperatures and comprise an oriented polypropylene film which contains at least one high-temperature sealable layer of an ethylene/propylene copolymer comprised of 2 to 6% by weight of ethylene and 98 to 94% by weight of propylene. These films have good high-temperature sealability, but they are not clear and scratch-resistant to the desired degree and, moreover, have inadequate processing characteristics on high-speed packaging machines.
EP-B-027,586 discloses sealable polypropylene films which have a sealing layer comprised of an ethylene homopolymer or copolymer, a long-chain aliphatic amine, an incompatible thermoplastic component, and a polydialkylsiloxane. These films represent an improvement over the films of DE-A-16 94 694, but they still do not run reliably on horizontal molding/filling/sealing machines.
DE-A-29 41 140 discloses a packaging material which is comprised of a base layer of a polypropylene polymer and of a surface layer which comprises a mixture of a propylene/ethylene copolymer and a (C.sub.4 -C.sub.10)-.alpha.-olefin/propylene copolymer. This surface layer can also contain a thermoplastic resin of low molecular weight, as well as silicone oils. A disadvantage of packaging materials of this type is that they are susceptible to scratching and have unsatisfactory optical characteristics.
EP 114,312 discloses a transparent polyolefin multilayer film, which is comprised of a base layer of a propylene polymer and at least one sealable layer. The sealable layer comprises an olefin resin composition, an ethylene/propylene/butylene polymer and a propylene/butylene polymer and contains 5-15% by weight of a compatible hydrocarbon resin, 5-15% by weight of propylene homopolymer and 0.3-1.5% by weight of polydiorganosiloxane. The film has a low sealing initiation temperature and runs reliably on high-speed packaging machines. However, the film is not printable. Moreover, because of the addition of resin, resin deposits are formed on the longitudinal drawing rollers during film production and this has an adverse effect on the optical characteristics of the film caused by striation. When Example 1 of EP 114,312 was repeated, it was also found that the film had deficiencies in frictional characteristics and poor optical characteristics including high turbidity and low gloss. Moreover, flow defects arise over the film width, which lead to a high rejection rate.
Furthermore, in their brochure TAFMER XR, Japan 82 03 1000 Cl, Mitsui Petrochemical Industries Ltd. describe an .alpha.-olefin copolymer which is suitable as a sealing layer for polypropylene films. However, as Comparison Example 8 of this application shows (see Table 1), polypropylene films provided with this sealing layer are not satisfactory in various respects.
None of the known multilayer films simultaneously meets the criteria which are important for a packaging film, i.e., low sealing temperature on both sides in combination with the following characteristics: printability, high surface gloss, low turbidity, homogeneous film optical characteristics (no striations, no flow defects), low susceptibility to scratching, good frictional characteristics and good running characteristics on high-speed packaging machines of various types.